Dry adhesion is the result of multiple attractive forces, including short-range intermolecular forces such as van der Waals' and longer range electrostatic forces, which occur between contacting materials. The total force generated between contacting materials is therefore directly related to the amount of area in intimate contact, as well as the chemical makeup of the mating surfaces. Substantial investments in the study and development of dry adhesives have been made in recent years, spurred in large part by the discovery of the exceptional adhesive capabilities of gecko lizards. The majority of this work has focused on the study of fibrillar or “hairy” dry adhesives, such as the naturally occurring gecko, and the development and characterization of artificial fibrillar systems meant to mimic those found naturally.
A compliant adhesive surface facilitates the creation of large adhesive contact area during initial contact (bonding). During loading, excessive compliance in the adhesive system may cause inadequate load sharing between contact points, leaving the interface susceptible to peeling failure. Therefore, strong adhesion may be generated by an adhesive surface that maximizes compliance normal to the mating surface during bonding, while also minimizing compliance in the direction of loading. Fibrillar structures may be one possible solution to this problem; the slender fibrillar structures maximize compliance at the microscale to generate a large contact area, while the underlying structure supplies adequate rigidity to suppress peeling. The large scale application of artificial fibrillar dry adhesives faces multiple significant challenges, however, most notably high fabrication costs and limited durability.
Alternative dry adhesive systems using similar principles of compliance control are thus being developed. For example, various researchers are investigating the use of phase-changing or smart materials, such as thermosensitive shape memory polymers (SMPs) for dry adhesive applications. A drawback to the use of thermosensitive functional materials is the need for a heat source to induce the temperature changes needed to transition between adhesive and non-adhesive states. An external heat source constitutes additional equipment cost and reduced flexibility of operation for the adhesive system, making the bonding process more complex and adding thermal mass, thus slowing the thermal response time of the functional material with a given power input.